The present invention relates to a method of manufacturing a large-sized, thin-walled, elongated object, i.e. a molding, of fiber reinforced, hardenable synthetic resin. The invention more particularly relates to such a method for the manufacture of a molding which is of such large size that the molding cannot be carried out by conventional injection molding machines, blow molding machines, vacuum molding machines, and the like, but must be carried out by human labor placing reinforcing fibers against a mold surface, with subsequent closing of the mold and flowing hardenable resin therethrough. Such large-sized moldings include boat hulls, windmill rotor blades, motor coach bodies, and the like.
In the prior art, moldings of this nature were carried out by laying at least one layer of reinforcing fibers against a form-retaining, rigid inner mold part, such that the layer is substantially co-extensive with the inner mold part. That laying of the fiber layer against the inner mold part (which may actually consist of a plurality of layers of fibers) is, usually, done by human hands. The layer is normally substantially coextensive with the inner mold part, although in this regard, for special moldings that fiber layer need not be completely coextensive with the inner mold part. For example, the inner fiber layer may have openings which are intended to become, for example, passageways in a boat or motor coach housing, or equipment openings or portholes and the like. Nevertheless, in the more usual sense, the fiber layer is substantially coextensive with the inner mold part.
After the fiber layer is laid against the inner mold part, a form-retaining, rigid outer mold part is placed against the fiber layer so as to form a mold cavity between those two mold parts. Usually, the inner mold part is in the general form of a female mold part and the outer mold part is generally in the form of a male mold part, although for certain moldings, the reverse may be true. In any event, the two form-retaining, rigid mold parts form a mold cavity with the fiber layer disposed within that mold cavity.
Thereafter, a liquid, hardenable synthetic resin is flowed into the mold cavity through one or more openings in at least one of the mold parts, to at least substantially fill the mold cavity, and usually entirely fill the mold cavity. In this regard, there are a number of techniques for filling the mold cavity with the synthetic resin which have been used in the art. For example, some of those techniques include flowing the resin through the mold cavity in such a volume so as to exceed the volume of the mold cavity, so that some resin overflows the mold cavity and, consequently, carries some entrapped air within the fiber layer and mold cavity out of the mold cavity. However, in all of the techniques, the mold cavity is at least substantially filled with the synthetic resin.
The general technique of filling the mold cavity is that of flowing the synthetic resin through one or more openings in at least one of the mold parts. The openings may be in the inner mold part or the outer mold part or they may be in the juncture between the two mold parts. Also, more usually, there is at least one opening at a lower part of the mold, so that the resin flows upwardly through the mold and through the fiber layer, and, consequently, will remove more air trapped within the mold cavity and fiber layer as the resin moves through the mold cavity.
As noted above, the general methods in the art for filling the mold cavity have, as a purpose, the removal of some of the air trapped within the mold cavity and the fiber layer. However, that flow of resin, even an upwardly flow of resin, is not sufficient to remove all, or even most, of the air trapped in the mold cavity and fiber layer, and the art, generally, attempts to remove further air from the mold cavity and fiber layer by inducing a reduced pressure within the mold cavity. This reduced pressure will pull trapped air from the mold cavity and the fiber layer out of the mold cavity. It is, of course, well recognized in the art that it is imperative to remove as much of the trapped air as possible from the mold cavity and fiber layer, since any air trapped in the molding itself will constitute voids, and large voids or a concentration of voids may cause potential weak spots in the molding.
To achieve the reduce pressure, at least one vent is placed in at least one of the mold parts and is connected to a vacuum source sufficient to remove air from within the mold cavity and the fiber layer. This reduced pressure will also remove some of the air trapped in the resin flowing in the mold. In this regard, as the resin is flowed through the mold cavity, the liquid resin has a tendency to entrain air, and the reduced pressure will pull some of that entrained air from the resin, as well as from the mold cavity and the fiber layer. The vent or vents may be in either the mold parts, or formed at a juncture between the mold parts (constituting a vent, in part, in each of the mold parts) and at least one or more vents is generally placed at an upper portion of the mold cavity, so as to more efficiently remove air from the mold cavity, fiber layer and resin.
The vacuum source which achieves the reduced pressure in the mold cavity is, generally, not a relative high vacuum source. As can be appreciated, both the inner and outer mold parts are form-retaining, rigid mold parts, in order to provide the mold definition required for the molding. Since the surface area of the two mold parts is quite large for such large-sized moldings, high vacuum sources and high reduced pressure cannot be tolerated. Otherwise, one or both of the mold parts would deform under the reduced pressure, and, thus, disrupt the required mold definition.
After the mold cavity has been filled with resin, and the reduced pressure within the mold cavity withdraws such air as is capable of being withdrawn, the reduced pressure is continued, and the filled mold is left at rest, for solidification of the hardenable resin. In this regard, typical resins are epoxy resins, polyester resins, and vinylester resins. Other hardenable resins may be used, depending upon the particular molding involved, but the foregoing are the more normal resins used in such large size moldings.
While resins that cure at elevated temperatures may be used, e.g. up to temperatures up to and in excess of 100.degree. C., these resins are more difficult to use, since the resin must be cured with added heat, e.g. large ovens or heat lamps. Therefore, more usually, these resins are room-temperature cure resins, which is usually defined as curable at a temperature between about 15.degree. C. and 30.degree. C. It is preferred that the liquid resin be a room-temperature cure resin, since moldings of this large size are often performed in the outside ambient weather or performed in large buildings where only about room temperature, e.g. 15.degree. to 30.degree. C., can be maintained.
The time for solidifying the resin, i.e. initially curing the resin, can vary with the particular resin, the catalyst used, and the ambient or heated temperature, but generally speaking the initial curing time is at least an hour, and more usually four to six hours, but may extend for longer times. For many resins, with particular catalyst, in order for the resin to completely cure, some days are required, but after the initial curing, i.e. solidification, of the resin, at least the inner mold part may be removed.
As can be appreciated from the prior art method of molding such large, thin-walled moldings, such as boat hulls and the like, it is necessary to have two form-retaining, rigid mold parts, i.e. the inner mold part and the outer mold part. The contours of these two mold parts must be complementary in order to achieve the required mold definition therebetween. Since these mold parts are quite large, it is also obvious that the cost of manufacturing these mold parts is also quite high. Further, since the mold definition between the two mold parts depends upon the retaining of the contours of these rigid mold parts, the degree of reduced pressure in the mold cavity should be carefully controlled, since an inappropriate reduced pressure could cause one or both of the mold parts to buckle and spoil the required mold definition. With low reduced pressures, it is difficult to remove all or most of the air from the mold cavity, fiber layer and resin, and often quite undesired voids in the molding results from the trapped air, and these voids, in some cases, may constitute potential weak portions of the molding, while with high reduced pressures buckling of a mold part may result.
Further, since two rigid mold parts are used, there is no possibility of compressing the fiber layer therebetween. This results in relatively low fiber-to-resin ratios, e.g. 3.5 to 4.5 fiber/6.5 to 5.5 resin. Most of the strength of the molding is from the fiber, and, hence, the lower ratios result in lower strengths of the molding.
In the above-noted parent application, the entire disclosure of which is incorporated herein by reference and relied upon for disclosure, there is disclosed a method of manufacturing such large-sized thin-walled, elongated moldings wherein the outer mold part is a non-form-retaining, flexible outer mold part, e.g. a flexible sheet or film made of plastic. When such flexible outer mold part is used, the reduced pressure tightly draws that flexible outer mold part against the fiber layer, and therefore the fiber layer functions to provide the required mold definition between the inner mold part and the outer mold part. This also compresses the fiber and results in a much higher fiber/resin ratio. With this arrangement, careful control of the reduced pressure is not required.
That application also discloses elongated cores which extends in the longitudinal direction of the molding. The cores have at least one resin supply duct extending in the longitudinal direction of the core, which duct functions to allow the liquid resin to flow therethrough and distribute that resin in the fiber layer, while the flexible outer mold part is tightly pulled against the fiber layer and toward the inner mold by means of the reduced pressure.
That application also discloses the disposing of a plurality of strips having longitudinal passageways in a direction substantially transverse to the direction of the cores such that the strips will further cause distribution of the resin from the cores and into the fiber layer.
Thus, the molding process described in the parent application is a substantial advance in the art of manufacturing such large-sized, thin-walled, elongated moldings. In one aspect of the present invention, it has been found, however, that the cores and strips described in the parent application do not distribute the resin in the fiber layer to the uniformity that would be desired. Thus, in this aspect of the present invention, a method is provided for more uniformly distributing the resin in the fiber layer.